Part 10 (2/2)
_E_. A mine once started on the system is most difficult to alter, owing to the lack of frequent winzes or pa.s.ses. Especially is this so if the only alternative is filling, for an alteration to the system of filling coincident with breaking finds the mine short of filling winzes. As the conditions of walls and ore often alter with depth, change of system may be necessary and the situation may become very embarra.s.sing.
_F_. The restoping of the walls for lower-grade ore at a later period is impossible, for the walls of the stope will be crushed, or, if filled with waste, will usually crush when it is drawn off to send to a lower stope.
The system has much to recommend it where conditions are favorable.
Like all other alternative methods of mining, it requires the most careful study in the light of the special conditions involved. In many mines it can be used for some stopes where not adaptable generally.
It often solves the problem of blind ore-bodies, for they can by this means be frequently worked with an opening underneath only.
Thus the cost of driving a roadway overhead is avoided, which would be required if timber or coincident filling were the alternatives.
In such cases ventilation can be managed without an opening above, by so directing the current of air that it will rise through a winze from the level below, flow along the stope and into the level again at the further end of the stope through another winze.
[Ill.u.s.tration: Fig. 40.--Longitudinal section. Ore-pillar support in narrow stopes.]
SUPPORT BY PILLARS OF ORE.--As a method of mining metals of the sort under discussion, the use of ore-pillars except in conjunction with some other means of support has no general application. To use them without a.s.sistance implies walls sufficiently strong to hold between pillars; to leave them permanently anywhere implies that the ore abandoned would not repay the labor and the material of a subst.i.tute. There are cases of large, very low-grade mines where to abandon one-half the ore as pillars is more profitable than total extraction, but the margin of payability in such ore must be very, very narrow. Unpayable spots are always left as pillars, for obvious reasons. Permanent ore-pillars as an adjunct to other methods of support are in use. Such are the rib-pillars in the Alaska Treadwell, the form of which is indicated by the upward extension of the pillars adjacent to the winzes, shown in Figure 37. Always a careful balance must be cast as to the value of the ore left, and as to the cost of a subst.i.tute, because every ore-pillar can be removed at some outlay. Temporary pillars are not unusual, particularly to protect roadways and shafts. They are, when left for these purposes, removed ultimately, usually by beginning at the farther end and working back to the final exit.
[Ill.u.s.tration: Fig. 41.--Horizontal plan at levels of Broken Hill.
Method of alternate stopes and ore-pillars.]
[Ill.u.s.tration: Fig. 42.--Longitudinal section of Figure 41.]
A form of temporary ore-pillars in very wide deposits is made use of in conjunction with both filling and timbering (Figs. 37, 39, 40). In the use of temporary pillars for ore-bodies 100 to 250 feet wide at Broken Hill, stopes are carried up at right angles to the strike, each fifty feet wide and clear across the ore-body (Figs. 41 and 42). A solid pillar of the same width is left in the first instance between adjacent stopes, and the initial series of stopes are walled with one square-set on the sides as the stope is broken upward. The room between these two lines of sets is filled with waste alternating with ore-breaking in the usual filling method.
When the ore from the first group of alternate stopes (_ABC_, Fig.
42) is completely removed, the pillars are stoped out and replaced with waste. The square-sets of the first set of stopes thus become the boundaries of the second set. Entry and ventilation are obtained through these lines of square-sets, and the ore is pa.s.sed out of the stopes through them.
[Ill.u.s.tration: Fig. 43.--Cross-section of stull support with waste reenforcement.]
ARTIFICIAL PILLARS.--This system also implies a roof so strong as not to demand continuous support. Artificial pillars are built in many different ways. The method most current in fairly narrow deposits is to reenforce stulls by packing waste above them (Figs.
43 and 44). Not only is it thus possible to economize in stulls by using the waste which acc.u.mulates underground, but the principle applies also to cases where the stulls alone are not sufficient support, and yet where complete filling or square-setting is unnecessary. When the conditions are propitious for this method, it has the comparative advantage over timber systems of saving timber, and over filling systems of saving imported filling. Moreover, these constructions being pillar-shaped (Fig. 44), the intervals between them provide outlets for broken ore, and specially built pa.s.ses are unnecessary. The method has two disadvantages as against the square-set or filling process, in that more staging must be provided from which to work, and in stopes over six feet the erection of machine-drill columns is tedious and costly in time and wages.
[Ill.u.s.tration: Fig. 44.--Longitudinal section of stull and waste pillars.]
In wide deposits of markedly flat, irregular ore-bodies, where a definite system is difficult and where timber is expensive, cribs of cord-wood or logs filled with waste after the order shown in Figure 31, often make fairly sound pillars. They will not last indefinitely and are best adapted to the temporary support of the ore-roof pending filling. The increased difficulty in setting up machine drills in such stopes adds to the breaking costs,--often enough to warrant another method of support.
[Ill.u.s.tration: Fig. 45.--Sublevel caving system.]
CAVING SYSTEMS.--This method, with variations, has been applied to large iron deposits, to the Kimberley diamond mines, to some copper mines, but in general it has little application to the metal mines under consideration, as few ore-bodies are of sufficiently large horizontal area. The system is dependent upon a large area of loose or ”heavy” ground pressing directly on the ore with weight, such that if the ore be cut into pillars, these will crush. The details of the system vary, but in general the _modus operandi_ is to prepare roadways through the ore, and from the roadways to put rises, from which sublevels are driven close under the floating ma.s.s of waste and ore,--sometimes called the ”matte” (Fig. 45).
The pillars between these sublevels are then cut away until the weight above crushes them down. When all the crushed ore which can be safely reached is extracted, retreat is made and another series of subopenings is then driven close under the ”matte.” The pillar is reduced until it crushes and the operation is repeated.
Eventually the bottom strata of the ”matte” become largely ore, and a sort of equilibrium is reached when there is not much loss in this direction. ”Top slicing” is a variation of the above method by carrying a horizontal stope from the rises immediately under the matte, supporting the floating material with timber. At Kimberley the system is varied in that galleries are run out to the edge of the diamond-iferous area and enlarged until the pillar between crushes.
In the caving methods, between 40 and 50% of the ore is removed by the preliminary openings, and as they are all headings of some sort, the average cost per ton of this particular ore is higher than by ordinary stoping methods. On the other hand, the remaining 50 to 60% of the ore costs nothing to break, and the average cost is often remarkably low. As said, the system implies bodies of large horizontal area. They must start near enough to the surface that the whole superinc.u.mbent ma.s.s may cave and give crus.h.i.+ng weight, or the immediately overhanging roof must easily cave. All of these are conditions not often met with in mines of the character under review.
CHAPTER XII.
Mechanical Equipment.
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